"Go ahead. Take a peek. It’s alright. We weren't quite ready, but we will
be soon, and we can’t wait to finally show you what we've really been
up to. Stick around, we haven’t even started yet."

The leak comes from a document created by an advertising agency that was put together as a presentation for Destiny and had the following description for the game:

“Our story begins seven hundred years from now in the Last City on Earth, in a Solar System littered with the ruins of man’s Golden Age. A massive, mysterious alien ship hangs overhead like a second Moon. No one knows where it came from or what it’s here for, but only that it’s our protector. Meanwhile, strange, alien monsters creep in from the edge of the universe, determined to take Earth and the Last City. We are young ‘knights’ tasked with defending the remains of humanity, discovering the source of these monsters and – eventually – overcoming it.”

The advertising document goes on to refer to the, "massive mysterious alien ship," as the Traveler, and says that Bungie is seeking to make Destiny as universally accessible as the Star Wars universe. Bungie's co-founder Jason Jones described Destiny saying it, "is designed for your inner seven year old. We want to make it feel like a mythic adventure.” Bungie has pointed out in that past that one of the reasons it decided to work with Activision was because of its impressive online infrastructure leading many to theorize that Bungie's next game may be an MMO. As part of the leak, the gameplay of Destiny is described as “social at its core,” and has “a world to explore with friends, both old and new,” which lends a little more ammunition to that rumor.

IGN has handful of images from the game on its site, but notes that many of the images are mock-ups from the advertising agency presumably based on Destiny that weren't necessarily created by Bungie.